The Paris Metro has become a cavern of ephemeral but beautiful underground art, discovered one month and destroyed the next. Renovations in two-thirds of the underground stations in the French capital have exposed a cornucopia of old, torn advertising posters or paintings. Some appear to date back to the earliest days of the Paris Metro a century ago.

At the George V station on the Champs Elysées, there is even the ragged remains of a poster warning, in German, of the dangers of feindagenten (enemy agents) at work on the streets of Paris. A few inches away are the ghostly remnants of a poster advertising Margaret Lockwood and Phyllis Calvert – glamorous British actresses of the 1940s – in the movie L'Homme en Gris (The Man in Grey).

The film was made in 1943 but, for obvious reasons, did not reach Paris until late the following year. In 1943, Ms Lockwood and Ms Calvert were still "feindagenten".

A little way up the platform area are a pair of disembodied duck's feet against a shattered green background with beautiful lettering in the style of the 1920s. That is the remains of a poster for the long-lost Parisian butcher and charcuterie, Buron Frères.

At George V, there is also the elegantly faded outline of an advertisement for "machines électriques" which appears to have been painted directly on to the wall. The George V station was one of the first to open in Paris, in 1900.

The tattered posters – often superimposed one over the other in fantastic collages – have been uncovered as part of a huge renovation programme by the Régie Autonome des Transports Parisiens (RATP), which runs the metro trains, buses and trams in the French capital. Two hundred of the 297 Metro stations are being modernised and refurbished. In some cases, false curving walls built on the station platforms in the 1960s and 1970s have been stripped away, revealing the jumble of old posters underneath.

Almost complete, though tattered, posters exist in some places. In others, there are fragments of layer after layer of old advertisements, forming works of abstract art which might easily command €1m (£800,000) at Sothebys or Christies.

Most Parisians, hurrying from home to work, cinema or restaurant, pay no attention to the ragged, transient beauty now displayed on the Metro walls. As The Independent was taking photographs, we were approached by Michel, 60, a retired travel agent and self-declared "passéiste" – a man obsessed with the past.

"I am pleased to see that you take an interest," he said. "Most people don't care. But just look how beautiful these old posters are. Think of all the generations of Parisians and visitors who have cast their eyes upon them."

A couple of websites have sought to fix some of the works of accidental art for posterity. One blogger, on francis75.canalblog.com, suggests that these "phantom-like visions" recall a scene from Frederico Fellini's film Fellini Roma.

Astonished workers constructing a tunnel discover ancient Roman frescoes underground, the blogger recalls. Almost as soon as they appear, the frescoes "disappear one by one, destroyed by exposure to the fresh air".

The ancient advertising frescoes are doomed to the same fate. Asked if there were any plans to preserve one or two of the more beautiful sites – "Les Sablons" and "Ecole Militaire" stations are also excellent – the RATP referred enquiries to Metro Bus, the company which runs the advertising space in Paris Metros and buses. A friendly woman in the Metro Bus technical department said: "Yes, many of these sites are extraordinary. Unfortunately, there are no plans to preserve any of these old posters. The RATP are not poets. They are a public transport company and committed to their renovation programme."

Some of the best sites have already been scrubbed clean to make way for the future.